It doesn't have a 100% win rate as it stands.

Mini-max isn't even 'playing safe'.

Consider the following choice of moves, each leading to one of 5 random tile inserts:

Move A, which leads to 5 possible moves with the following game state goodnesses: ['Loss','Win','Win','Win','Win']

Move B, resulting in: ['99%CertainLoss','99%CertainLoss','99%CertainLoss','99%CertainLoss','99%CertainLoss']

Minimax is never going to choose A. Is that really what you want, even if your strategy is 'play it safe'?